Persistence of motor adaptation during constrained, multi-joint, arm movements

We studied the stability of changes in motor performance associated with ad aptation to a novel dynamic environment during goal-directed movements of t he dominant arm. Eleven normal, human subjects made targeted reaching movem ents in the horizontal plane while holding the handle of a two-joint roboti c manipulator. This robot was programmed to generate a novel viscous force field that perturbed the limb perpendicular to the desired direction of mov ement. Following adaptation to this force field, we sought to determine the relative role of kinematic errors and dynamic criteria in promoting recove ry from the adapted state. In particular, we compared kinematic and dynamic measures of performance when kinematic errors were allowed to occur after removal of the viscous fields, or prevented by imposing a simulated, mechan ical "channel" on movements. Hand forces recorded at the handle revealed th at when kinematic errors were prevented from occurring by the application o f the channel, recovery from adaptation to the novel field was much slower compared with when kinematic aftereffects were allowed to take place. In pa rticular, when kinematic errors were prevented, subjects persisted in gener ating large forces that were unnecessary to generate an accurate reach. The magnitude of these forces decreased slowly over time, at a much slower rat e than when subjects were allowed to make kinematic errors. This finding pr ovides strong experimental evidence that both kinematic and dynamic criteri a influence motor adaptation, and that kinematic-dependent factors play a d ominant role in the rapid loss of adaptation after restoring the original d ynamics.